LightSail is a citizen-funded project by The Planetary Society, the world's largest non-profit space advocacy group. They are sending two small spacecraft into Earth orbit carrying large, reflective sails measuring 32 square meters (344 square feet). Their first mission is a May 2015 test flight that will pave the way for a second, full-fledged solar sailing demonstration in 2016.

Solar sails use the sun's energy as a method of propulsion—flight by light. Light is made of packets of energy called photons. While photons have no mass, a photon traveling as a packet of light has energy and momentum. Solar sail spacecraft capture light momentum with large, lightweight mirrored surfaces—sails. As light reflects off a sail, most of its momentum is transferred, pushing on the sail. The resulting acceleration is small, but continuous. Unlike chemical rockets that provide short bursts of thrust, solar sails thrust continuously and can reach higher speeds over time.

LightSail is a CubeSat. These tiny spacecraft often hitch rides to orbit aboard rockets carrying bigger payloads. CubeSats have standard unit sizes of 10 centimeters per side. They can be stacked together—LightSail is a three-unit CubeSat about the size of a loaf of bread.

Once in space, LightSail's solar arrays swing open, revealing the inside of the spacecraft. Four tape measure-like metal booms slowly unwind from storage, unfolding four triangular, Mylar sails. Each sail is just 4.5 microns thick—one-fourth the thickness of an average trash bag.

Three electromagnetic torque rods aboard LightSail will interact with Earth's magnetic field, orienting the spacecraft. Ground-based lasers will measure the effect of sunlight on the sails. As LightSail breezes around the Earth, its shiny sails will be visible from the ground.

The test flight is slated for May 20 when the first LightSail spacecraft will hitch a ride to orbit aboard an Atlas V rocket. It won't fly high enough above the Earth's atmosphere for solar sailing, but will test sail deployment sequence and take some photographs.

In 2016, LightSail will be enclosed within Prox-1, a small satellite developed by the Georgia Institute of Technology (Georgia Tech) to autonomously inspect other spacecraft. Both satellites will be lifted into orbit by the Falcon Heavy, a new heavy-lift rocket built by private spaceflight company SpaceX. LightSail and Prox-1 will be released into an orbit with an altitude of 720 kilometers (450 miles), high enough to escape most of the planet's atmospheric drag. Prox-1 will eject LightSail into open space. Later, it will rendezvous with LightSail and inspect it. When LightSail unfurls its solar sails, Prox-1 will be nearby to capture images of the big moment.

Source and top image: The Planetary Society

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